This disclosure relates to an energy management structure, and particularly to an energy management structure that is adapted to absorb an impact force applied thereto. The energy management structure is applicable, but not limited, to body protective gear for military, athletic and recreational activities, for example, including a helmet where a chance of cranial impact is possible, protective pads (e.g., shoulder pads), and other pads such as cushions for hard seats and packing material.
Helmets and other types of protective gear used in military, athletic and recreational activities are intended to provide protection against a sudden impact. Generally, a helmet protects the head by preventing direct impact on the head, but also by causing the head to more gradually accelerate to a stop. The shell of the helmet and liner components together act as a system. The shell is constructed to provide rigidity to the helmet to guard against protruding objects, and is as a platform to support the liner structure. The liner is designed to complement the shell's geometry and regional stiffness, so that the liner and shell together provide a more ideal reduction in peak accelerations that would be observed should a head come to violent and sudden stop or accelerated state. In an impact scenario, the shell and liner provide an appropriate efficiency and stiffness response such that the head is more gently accelerated using as much of the available liner thickness as possible. Herein, “acceleration” is used to describe both acceleration and deceleration.
Expanded Polystyrene (EPS), Expanded Polypropylene (EPP), Ethyl Vinyl Acetate (EVA), Vinyl Nitrile (VN), and Polyurethane foams (PU) are conventional materials used for acceleration management inside helmets to protect the head. Non-foam materials, such as honeycomb, hemisphere and cylindrical crushable structures have been suggested as an alternative for managing impact forces applied to the helmet. All of these materials and structures exhibit crushing efficiencies, typically less than 75% compression until the material is considered bottomed. Crushing efficiency of the materials and structures are affected by their density. Generally, greater density results in increased stiffness. As stiffness is increased, crush efficiency is diminished. Crushable structures also have inherent crushing efficiencies, which are ultimately governed by the stacking-up of the structure walls at full compression. Structures such as honeycombs can be very stiff, but typically not very efficient or resilient. In order to reach full compression, wall damage of these structures is often observed.